Vital signs of a person, for example the heart rate (HR), the respiration rate (RR) or the arterial blood oxygen saturation (SpO2), serve as indicators of the current health state of a person and as powerful predictors of serious medical events. For this reason, vital signs are extensively monitored in inpatient and outpatient care settings, at home or in further health, leisure and fitness settings.
One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave traveling through the body of a subject with every heartbeat.
Photoplethysmography (PPG) is an optical measurement technique that evaluates a time-variant change of light reflectance or transmission of an area or volume of interest. PPG is based on the principle that blood absorbs light more than surrounding tissue, so variations in blood volume with every heart beat affect transmission or reflectance correspondingly. Besides information about the heart rate, a PPG waveform can comprise information attributable to further physiological phenomena such as the respiration. By evaluating the transmittance and/or reflectivity at different wavelengths (typically red and infrared), the blood oxygen saturation (SpO2) can be determined. Different kinds of such contact sensor are commonly known and used, including contact finger pulse oximeters, contact forehead pulse oximeter sensors, contact pulse sensors, etc.
Recently, non-contact, remote photoplethysmography (rPPG) devices (also called camera PPG devices) for unobtrusive measurements have been described in many publications, e.g. in Verkruysse et al., “Remote plethysmographic imaging using ambient light”, Optics Express, 16(26), 22 Dec. 2008, pp. 21434-21445, which demonstrates that photoplethysmographic signals can be measured remotely using ambient light and a conventional consumer level video camera, using red, green and blue color channels.
Remote PPG utilizes light sources or, in general radiation sources, disposed remotely from the subject of interest. Similarly, also a detector, e.g., a camera or a photo detector, can be disposed remotely from the subject of interest, i.e. without contact to the subject. Therefore, remote photoplethysmographic systems and devices are considered unobtrusive and well suited for medical as well as non-medical everyday applications. This technology particularly has distinct advantages for patients with extreme skin sensitivity requiring vital signs monitoring such as Neonatal Intensive Care Unit (NICU) patients with extremely fragile skin or premature babies.
Current methods for evaluation of blood pressure changes are based on measurement of pulse transit time (PTT) or pulse arrival time (PAT). The first approach estimates the transit time between one signal carrying the arterial pulse wave (pulse wave signal) and another signal such as the electrocardiogram (ECG). The time interval between the ECG fiducial point (typically the R peak) and a fiducial point marking the pulse arrival is referred to as the PAT. The PTT is the time difference between the aortic valve opening and the pulse wave arrival. The second approach estimates the BP from the PTT between two pulse wave signals measured at different parts of the body.
These methods require placement of contact PPG (and/or ECG) sensors at two body locations, at least, preferably at large distance from each other. This might require significant time investment and efforts to estimate changes of blood pressure based on PTT. Moreover, in order to follow the trend of changes of blood pressure over long periods of time (e.g. days), the location for placement of contact sensors, as well as position of a body should be the same for every measurement. Furthermore, the two sensors needs to be synchronized to millisecond level in order to provide accurate PTT measurement. Finally, contact sensors are sensitive to motion of a person, and are sensitive to correct placement.
Those disadvantages of current methods of PTT measurement limit the use of such approach beyond professional healthcare environment (e.g. at home, on the go, etc.). Therefore, there is a need for a system, which can remove disadvantages of current systems for PTT-based measurement of pulse-related information such as e.g. blood pressure changes, pulse transit time and/or pulse arrival time.